Magnetic properties and magnetic signatures of BIFs of the Hamersley Basin and Yilgarn Block, Western Australia

Abstract

Magnetite-rich banded iron-formations (BIFs) exhibit characteristic magnetic properties, including strong anisotropy. Interpretation of the geological structure of BIF units from the associated magnetic anomalies is complicated by anisotropy of susceptibility and, frequently, by remanent magnetisation. The timing of remanence acquisition relative to folding exerts a crucial influence on the form of the anomalies. Neglect of susceptibility anisotropy and remanence can lead to large errors in interpreted dips and thicknesses of BIF units. The effective susceptibility of BIFs parallel to bedding exceeds the susceptibility normal to bedding, typically by a factor of 2 to 4. Bedding-parallel susceptibilities of magnetite-rich BIFs are typically 0.5 to 2.0 SI (0.05?0.16 G/Oe). Remanence directions in BIFs usually lie close to the bedding plane. Koenigsberger ratios (Q) of BIFs vary widely, but characteristic values can often be determined for individual units. Q values in the range 1 to 2 are common. The magnetisations of the haematite-rich supergene-enrichment iron ores are much lower than those of their BIF precursors. Magnetic properties of outcropping BIFs are usually greatly modified by weathering, which substantially decreases the bulk susceptibility, the degree of anisotropy and the remanence intensity. Deeper and more intense weathering of BIFs is encouraged by faulting and can be associated with reduced magnetic response over intensely faulted zones. The remanence of BIFs from the Hamersley Basin is carried by late diagenetic to low-grade metamorphic magnetite after primary haematite. At Wittenoom and Paraburdoo, the remanent magnetisation of Brockman Formation BIFs is pre-folding, whereas in an area of higher metamorphic grade, the Turner Syncline, the remanence of the BIFs is probably post-folding. Aeromagnetic signatures over the Turner Syncline clearly reflect anisotropy and remanence. Anisotropy and remanence effects are also evident in observed magnetic signatures over Archaean BIFs in the Yilgarn Block. Magnetic property measurements on samples from the Mount Magnet area and elsewhere confirm the high anisotropy and strong remanent magnetisations of these rocks.